The planet's parent star is a special kind of flashing star known
as a millisecond pulsar, a rapidly rotating neutron star formed
from a supernova. The entire system, which is only the second of
its kind ever discovered, is located about 4,000 light-years from
Earth in the constellation of Serpens (The Snake).

A gem of a find

Seventy percent of millisecond
pulsars found have a companion, which provides additional
energy to ramp up the pulsars' rapid rotation. Generally, this
companion is a dying star called a
white dwarf ; more than 180 millisecond pulsars have been
found with white dwarfs over the years.

Theonly planet known to be orbiting in such a system was detected
in 1992 — until now."The pulsar was found in December 2009," lead
scientist Matthew Bailes of Swinburne University of Technology in
Melbourne, Australia, told SPACE.com via email."We've been on the
trail of the companion ever since." [ The
Strangest Alien Planets ]

Known as PSR J1719-1438, this particular pulsar completes more
than 10,000 rotations in a minute. Tiny and compact, it's only
about 12 miles (20 kilometers) across, but it has a mass that is
1.4 times that of our sun.

PSR J1719-1438 transformed from an average star to a radio pulsar
when a dying star in a binary system exploded. The compact core
of the star formed with a very high rotation speed from the ashes
of the supernova.

When the second star in the system reached the end of its life,
it expanded as a
red giant and finally morphed into a white dwarf. The pulsar
began to suck mass off its companion, causing the pulsarto spin
faster and faster until it attained its breakneck speed.

From dying star to diamond planet

What happened next depends on the system. Most white dwarfs
continue to orbit the new millisecond pulsar, but some are
consumed by it.

"The fate depends upon the mass of the white dwarf and how far it
is from the pulsar," Bailes said.

If it is both close and massive, the two spiral together.
Astronomers assume this is what happened to the 30 percent of
millisecond pulsars found without a companion. [ Top 10 Star
Mysteries ]

In the case of the diamond planet, astronomers think that the
core of the white dwarf failed to merge completely with its
companion.

"When they got very close, the star lost a lot of its matter and
moved out to its safe distance of about a solar radius," Bailes
said.

Now tiny, having lost more than 99.9 percent of its original mass
and no longer engaged in the fusion reactions that drive a star,
the dead core is classified as a planet.

Ironically, the star-turned-planet is larger than its sun. With a
diameter of about 37,300 miles (60,000 km), it's five times the
size of Earth, but 3,000 times larger than the millisecond pulsar
it orbits.